Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory. An increase in the performance and density of memory devices is needed, for example, as the performance of computer systems increases. In certain instances, transistors have been reduced in size to accomplish density and performance increases, also resulting in increased speed with decreased power requirements. However, further reductions in size are often limited by the lower limit of dimensions that can be achieved using lithographic methods. Electron beam (e-beam) lithography and extreme ultraviolet (EUV) lithography have been used in attempts to prepare smaller features than are available using conventional lithographic methods. Although such methods can be capable of preparing smaller features, widespread use of such methods has been hampered by difficulties including, for example, high costs and/or incompatibility with high throughput production methods.
Self-assembly of block copolymers has also been used to prepare small features. For example, block copolymers, upon annealing, can undergo microphase separation to minimize contact between unlike domains. For example, the polymer molecules in an A-B block copolymer can be arranged in a head-to-head and tail-to-tail arrangement (e.g., A-B:B-A:A-B:B-A) to minimize contact between A and B blocks. As a result, regular patterns can be formed having the domain width (i.e., width of A:A or B:B, which is the average coil length of A:A or B:B) as the controlling feature size. Because at least some minimum average coil length or molecular weight for each block (A or B) is required to drive microphase separation, it becomes impractical to attempt to reduce feature size by reducing coil length or molecular weight for one or more of the blocks. For these as well as other reasons (e.g., blurring at the interface), patterns formed by self-assembly of block copolymers are practically limited to those having a domain width of at least 5 nanometers, with typical domain widths being substantially larger.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for new methods for preparing small features.